The oil ring used in the pistons of conventional reciprocating internal combustions typically consists of a three piece oil ring including a pair of annular side rails positioned one above the other and a space expander interposed between the two side rails. See JP2003-194222A, for instance.
In recent years, there has been an increasing desire to reduce the frictional resistance between the oil ring and the associated cylinder wall surface in view of improving fuel economy. The frictional resistance may be reduced by decreasing the pressure exerted on the cylinder wall surface by the oil ring, but it may impair the oil control function of the oil ring such as the scraping of the lubricating oil off the cylinder wall surface. Therefore, the pressure of the oil ring on the cylinder wall surface cannot be reduced beyond a certain limit.